“K. Watanabe, et al., “Development of MPO type 8-multicore fiber connector,” Optical Fiber Communications Conference and Exhibition (OFC) 2015, paper W4B. 3.” discloses a condition for polishing a connecting end face of an optical fiber for physically contacting multi-core fibers each having a plurality of cores, each other, and some experiment results of it. The connecting end faces of the multi-core fibers are polished to such a level that the difference between the maximum and the minimum amounts by which the cores protrude from the ferrule becomes equal to or less than 0.26 micrometer, and the connecting face of the multi-core fiber is then brought into contact with that of another by applying a compressive force. The experiment results indicate that, with such a polishing condition, the compressive force required for each one of the cores to be connected using physical-contact (PC) connection can be reduced to 14 N.